Infinitely variable traction roller transmission

ABSTRACT

In a traction roller transmission having toric discs mounted opposite each other on its input and output shafts motion transmitting traction rollers are engaged between the toric discs and supported by pivotal roller support structures permitting adjustment of the ratio of motion transmission between the input and output shafts. A cam structure is arranged adjacent a toric disc to provide an axial contact force depending on the torque transmitted through the transmission, the cam structure having cam areas with different leads, the cam leads adjacent the cam crests being larger than those of the cam surfaces adjacent the cam low points so as to reduce the contact forces provided by the cam structure when the cam areas with large lead are utilized during high transmission ratio pivot positions of the traction rollers.

BACKGROUND OF THE INVENTION

The invention relates to an infinitely variable traction rollertransmission in which motion is transmitted from an input toric disc toan output toric disc through traction rollers pivotally supportedbetween the toric discs, the discs being forced toward each other forengagement with the traction rollers therebetween.

Generally, the engagement forces are obtained by an axial cam structurewhich provides for an axial force dependent on the torque transmittedthrough the transmission. Such an arrangement is shown, for example, inU.S. Pat. No. 4,086,820, which is assigned to the assignee of thepresent application.

It has been found however that, in a high speed reduction mode, the camstructure produces contact forces far in excess of what is needed forslip-free power transmission since the traction rollers are then inengagement with the radially inner area of the input toric disc which,in this position, is wedged between the rollers by the axial forces. Theangle of contact at this point--with respect to the axis of the toricdisc--is an acute angle α so that the contact force Fc is substantiallylarger than the part of the axial cam force Fa₁ generating theparticular contact force, that is,

    Fc=Fa.sub.1 /sin α.

If α is, for example, 30° then

    Fc=Fa.sub.1 /1/2=2Fa.sub.1 ;

that is, the contact force is twice the corresponding cam force.Accordingly, greater contact forces than needed are generated, resultingin greater than necessary wear and friction, and furthermore in outwarddeflection of the traction roller support structures and also inexcessive cam lift and possible jumping of the cam rollers over the camcrests.

SUMMARY OF THE INVENTION

In a traction roller transmission in which toric traction discs aremounted opposite each other on input and output shafts with motiontransmitting traction rollers pivotally supported between, and inengagement with, the toric discs so as to permit adjustment of the ratioof motion transmission between the input and output toric discs, a camstructure is arranged adjacent a toric disc to provide an axial contactforce depending on the torque transmitted through the transmission. Thecam structure has cam surface areas with different leads, the cam leadsadjacent the cam crests being larger than those of the remainder of thecam surfaces so as to reduce the contact force provided by the camstructure when the cam areas with large leads are utilized during hightransmission ratio pivot positions of the traction rollers.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a traction roller transmission;

FIG. 2 shows schematically the traction rollers in a high transmissionratio position; and

FIG. 3 shows a cam structure for providing a roller engagement forcewhich is modified depending on the transmission ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, the traction roller transmission comprises a housing10 consisting of a central part 13, a front part 11 disposed at one endof the central part and a rear part 12 disposed at the opposite end ofthe central part 13. The housing parts 11, 12 and 13 are held togetherby bolts 14. Coaxial input and output shafts 15 and 16 extend throughthe front and the rear parts 11 and 12 of the housing 10 and arerotatably supported by input and output shaft bearings 17 and 18 and bycentral support bearings 19 and 20 or, alternatively, one shaft mayextend into a central bore in the other shaft to be supported therein.

The input shaft 15 carries an input traction disc 21 and the outputshaft 16 carries an output traction disc 22 disposed opposite the inputtraction disc 21. The traction discs 21 and 22 have opposite toroidalsurfaces 23 and 24 and are adapted to engage therebetween tractionrollers 25 and 26 for the transmission of motion from the input tractiondisc to the output traction disc. The traction rollers 25 and 26 aresupported by bearings 27 on a shaft 28 journalled in a roller supportstructure 29. The shaft 28 has slightly eccentric bearing portions 30and 40 to permit some translation of the rollers when necessary for firmengagement with the input and output traction discs. Axial support isprovided for the traction rollers 25 and 26 by axial thrust bearings andseal assemblies 31.

The output traction disc 22 is mounted on an axial output thrust member32 supported on the output shaft 16 for rotation therewith. Ahydrostatic axial thrust bearing and seal structure 34 is disposedbetween the axial thrust member 32 and the housing part 12 to provideaxial support for the thrust member 32 and the output traction disc 22.

The input traction disc 21 is mounted on an axial input thrust member 33which is freely rotatable on the input shaft 15 and, together with anaxial pressure plate 35, forms a load cam structure 37 for forcing theinput traction disc 21 toward the output traction disc 22 and both discsinto firm engagement with the traction rollers 25 and 26 when a torqueis transmitted through the transmission. The cam structure 37 has cams41 formed on the pressure plate 35 and cam faces or simply thrustsurfaces may be provided on the thrust member 33 with cam rollers 43disposed between the thrust member 33 and the cams 41 to be wedgedtherebetween when a torque is applied to the input shaft. The rollers 43are held in position by a cage 45.

FIG. 2 shows schematically input and output toric discs 21 and 22 withthe traction rollers 25 and 26 engaged therebetween in a hightransmission ratio position to indicate the forces: the axial cam forceA as generated by the cam structure 37 produces on each traction rollera contact force C which is normal to the roller surface in the contactarea and which increases as the angle α between the tangential plane inthe contact area and the toric disc axis decreases. With two tractionrollers, the traction roller contact force C is

    C=A/2 sin α

which becomes very large as α becomes small during high transmissionratio pivot positions of the traction rollers 25 and 26. These largecontact forces C generate large axial forces R on the traction rollerswhich are taken up by the traction roller support structures, however,not without deflection of the traction roller support structures. It isnoted that not only are large contact forces generated by apredetermined axial force during large transmission ratio pivotpositions of the traction rollers, but there is also a relatively largeamount of axial travel necessary for the toric disc 21 to generate thecontact forces since angle α is relatively small. Axial travel of thetoric disc is even further increased by a certain resiliency of thetraction roller support structures. A small angle α and resiliency ofthe roller support structure in combination may provide for such largeaxial cam lift that the cam rollers roll over the cam crests whichresults in failure of the transmission.

The contact forces generated with high transmission ratio pivotpositions of the traction rollers are actually far in excess of what isnecessary to transmit the desired torque and, being in excess of therequired value, they generate only greater friction and cause fasterwear of the traction surfaces.

To eliminate this problem, the present invention provides a camstructure with varying lead, that is, with a cam lead which increasestoward the cam crest 46. Such an arrangement is shown in FIG. 3. A camroller 43 is shown disposed between the cam surface 41 and the surfaceof thrust member 33 which, in the arrangement of FIG. 3, is alsoprovided with a cam surface corresponding to the cam surface 41.

During normal transmission ratio pivot positions of the traction rollers25 and 26, that is, with about a 1:1 transmission ratio, the cam roller43 is in a linear slope area of the cam structure which is so selectedas to provide contact forces large enough to avoid slipping of thetraction rollers. In high transmission ratio pivot positions where arelatively large cam lift is necessary because of a small angle α (FIG.2), the cam surfaces with increased lead will come into contact with thecam roller 43 providing a reduced axial contact force and increasedlift. However, the cam lead near these end positions is so selected thatthe contact forces at the traction rollers are still sufficiently largeto prevent slippage.

If, as shown in FIG. 3, cam surfaces are provided at both sides of thecam structure, engagement forces are generated upon relative motion ofthe cam surfaces in either direction so that the transmission may beoperated in either direction of rotation.

With this simple arrangement, the traction roller engagement forcescannot only be limited to the value necessary for firm engagement of thetraction rollers with the toric discs, it is furthermore practicallyimpossible for the cam rollers to roll over the cam crests so thatfailure will not occur already after a relatively small amount of wearhas occurred. In fact, wear of the surfaces in engagement in hightransmission ratio pivot positions will also be greatly reduced.

I claim:
 1. An infinitely variable traction roller transmissioncomprising: coaxial input and output shafts; oppositely disposed torictraction discs, one being supported by each of said shafts, for rotationtherewith; at least two motion transmitting traction rollers arranged inradial symmetry with respect to the axis of the input and output shaftsand in engagement with said toric discs for the transmission of motiontherebetween; a pivotal support structure for each of said tractionrollers permitting a change of the ratio of motion transmission betweenthe toric traction discs; and an axial cam structure disposed adjacentat least one of said toric traction discs for forcing said one torictraction disc toward the other so as to cause firm engagement of thetraction rollers with the traction discs when a torque is transmittedthrough said transmission, said cam structure including cams having mainoperating surface areas with cam crests therebetween, said mainoperating surface areas having a lead so selected as to provide an axialcontact force sufficient to cause firm engagement of said tractionrollers with said toric discs during normal transmission ratios when thetraction rollers axes are about normal to the axis of the toric tractiondiscs and having areas with increased slopes adjacent the crests so asto provide for increased leads in the areas adjacent the crests therebyto reduce the axial contact force in high transmission ratio pivotpositions of traction rollers.
 2. An infinitely variable traction rollertransmission as claimed in claim 1, wherein said cam structure includesantifriction bearing elements disposed to roll on said cam surfaces. 3.An infinitely variable traction roller transmission as claimed in claim2, wherein said antifriction bearing elements are rollers and said camsurface areas with increased lead adjacent said crests are curved havinga radius of curvature larger than the radius of said rollers.
 4. Aninfinitely variable traction roller transmission as claimed in claim 1,wherein cam areas are arranged at opposite sides of said antifrictionbearing elements.